Oval Port Heads vs Rectangle

Life is full of constants. The sun will rise each morning, Newton’s apple will fall, and if you are going to make power with a big-block Chevy, you have to use rectangle-port cylinder heads. So honored is this tradition among Chevy enthusiasts that many would rather face death and taxes than install a set of oval port heads. The reasoning is sound and not without precedence, as every factory high-performance big-block came equipped with rectangular-port heads. The oval-port variety was left for more pedestrian people movers and (of all things) high-torque truck applications. No self-respecting L72, L88, or LS6 owner would be caught dead sporting a set of oval port heads on his high-performance machinery. That Chevy further downgraded the oval port heads to peanut-port status is a clear indication that maximum performance was never high on the list of design criteria. Now that we all agree on the superiority of rectangular-port heads, the only thing left to do is convince you that oval port heads are actually the hot setup for the vast majority of street/strip big-blocks.

Big-block enthusiasts have always looked down their noses at oval-ports, but we’re here to tell you that round is the new square. As a general rule, round (or oval) ports will outflow their rectangular counterpart given equal dimensional area. The reason is that the corners of a rectangle are actually wasted space in terms of flow. The flow rate in the corners is very limited, so the wasted space does little more than increase port volume. This is obviously not a desirable situation, as power production is a product of both airflow and volume. The ideal situation (for most street/strip motors) is to maximize flow while minimizing port volume. There’s an argument to be made for increased port volume on high-rpm (dedicated) race motors, but for most, small port and big flow is the best combination. Looking strictly at flow, we see a direct correlation between port volume and flow on the factory BBC heads. The smallest of the bunch (peanut port) flows the least at around 250 cfm, while the conventional oval-ports flow around 275 cfm. The performance-oriented, rectangular-port heads step these numbers up above 300 cfm, but flow (as we know) is the equivalent of go.

When it comes to the amount of power a cylinder head will support, a good rule of thumb is to double the maximum airflow. Most (mild-cammed) street engines never achieve this level, while dedicated race motors can exceed this, but it’s accurate nonetheless and makes for easy math. Using this simple formula, we see that the factory heads can support 500 hp (peanut-port), 550 hp (oval-port), and over 600 hp (rectangular port). Ported versions obviously have the potential to exceed these numbers, but it shows a well-established trend in head selection. If you’re looking to exceed 600 hp, the obvious choice is the rectangular port, but what about all the other big-block buildups that range from 400–550 hp? These would likely be better served with the smaller oval-port heads, but our comparison doesn’t stop there. So far we have examined only the factory offerings, but how many buildups actually rely on stock castings? Given the number of aftermarket head choices available for the BBC, more often than not the factory stuff gets ditched in favor of something superior.

The introduction of performance castings by the aftermarket has greatly blurred the distinction between rectangular- and oval-port heads. Current aftermarket oval-port heads (like the AFR 265s tested here) not only match or exceed the flow offered by a set of stock rectangular-port heads, they do so with considerably less port volume (265 cc versus 310–320 cc for stock heads). More flow is always welcome, but even more so when it comes with a reduction in port volume. The combination equates to increased cylinder filling combined with improved low-speed and part-throttle response. Fuel mileage will likely increase as well, though the number of times you stomp on the loud pedal will have more of an effect on your actual fuel economy. The peak airflow offered by the AFR 265 heads (332 cfm) suggests they would support nearly 700 hp on the right application. That is a far cry from the factory ratings of the rectangular-port headed L72, LS6, or even L88s of yesteryear.

Since man does not live by flow numbers alone, we decided to take the cylinder head comparison to the next level. In addition to flow-bench numbers, we decided to compare a set of factory rectangular-port heads to the oval-port AFRs on the engine dyno. Sure, street and strip testing would be the ultimate follow up, but the dyno should illustrate the correlation between head flow, port volume, and power. Since the airflow/horsepower formula only hints at potential power (the whole combination must be optimized), we decided a real-world test on a streetable big-block was the hot setup. Though the airflow offered by the AFR 265s will certainly support a 496 stroker, we decided to test the heads on a 454 (actually, 0.060-over 468). The 468 featured machining and balancing from L&R Automotive, forged crank and rods from Procomp Electronics, and forged (18cc dome) pistons from Probe Racing. Combined with the 119cc chambers on the rec-port heads, the pistons produced a static compression of 9.70:1. This was increased to 10.47:1 with the 112cc chambers employed on the AFR heads. Given that every point of compression is worth 3 to 4 percent, the change in chamber volume was worth about 2.5 percent.

Cam timing is important for a couple of reasons, not the least of which is that it determines the effective operating range of the motor. It is also a limiting factor in terms of potential power since it determines the actual airflow employed by the motor. If your heads flow 350 cfm at 0.700 lift but your cam is only 0.600 lift, then you’re not taking full advantage of all the airflow available from your cylinder heads. Naturally, this limits the power output of your motor. For our test motor, we chose a streetable, but powerful, hydraulic roller cam. The XR294HR offered a 0.540/0.560 lift split, a 242/248 duration split and a 110-degree lobe-separation angle. This was a healthy cam to be sure for a 468, but we wanted to ensure our test motor was powerful enough to properly test the merits of the AFR oval-port heads. Comp Cams also supplied a set of roller rockers and hardened pushrods, while the various heads required dedicated intakes. We chose single-plane intakes for testing, but also ran the AFR heads with a dual-plane RPM Air Gap to illustrate the benefits offered for a dedicated street application. The stock rec-port heads received a Weiand Team G, while the AFRs were run with an Edelbrock Victor Jr. For many street/strip motors, the dual plane will offer improved torque production through most of the rpm range (see dyno results), but the cam was a better match for the single plane (at least for peak power production).

Though we have covered the major components employed on the test motor, the 468 also featured Fel Pro head gaskets, ARP head studs, an MSD billet distributor, and Holley 950 HP carburetor. All testing was run with a Meziere electric water pump and a set of 2.125-inch dyno headers.

First up were the stock rectangular-port heads (088 casting number). The open-chamber, rec-port heads featured 119cc chambers, a 2.19/1.88 valve combination and a valvespring upgrade for use with the hydraulic roller cam. As luck would have it, both the stock and AFR castings utilized the same (8.70/7.75-inch) pushrod combo. After dialing in ignition timing and air/fuel ratio, the rec-port 468 eventually pumped out 541 hp and 513 lb-ft of torque. Torque production exceeded 500 lb-ft from 4,400 rpm to 5,500 rpm, and the 119 cc intake ports offered 489 lb-ft of torque at 3,300 rpm. The maximum airflow of 334 cfm offered by the rec-port heads suggested they would support well over 650 hp, but the dyno doesn’t give a rat’s ass about airflow numbers and potential power. That’s why airflow numbers only provide a small part of the performance equation. With nearly identical peak intake flow numbers, how would the AFR oval-port heads fair on the big-block?

2/11After minor jetting and timing sweeps, the iron heads produced peaks of 541 hp and 513 lb-ft of torque. Torque production with the rec-port heads exceeded 500 lb-ft from 4,440–5,500 rpm.

Installation of the AFR 265 oval-port heads went without a hitch. The heads were supplied with sufficient valvespring pressure to allow the test motor to rev cleanly past 6,500 rpm without concern for valve float or bounce. Some will point to the change in compression ratio, and some to the port-matched intake manifold, but the reality is that the AFR 265 heads simply kicked the crap out of the stock rec-port heads. How much power was the head swap worth? Equipped with AFR’s 265 oval-port heads, the 468 produced 622 hp at 6,400 rpm and 569 lb-ft of torque at 5,200 rpm. Where torque production exceeded 500 lb-ft from 4,400 rpm to 5,500 rpm with the rec-port heads, the AFR head swap extended this range from 3,200 rpm (possibly lower) to 6,500 rpm. Measured peak to peak, the AFR heads improved power output by 81 hp and 56 lb-ft of torque, but the gains exceeded 90 hp higher in the rev range. Toss in the fact that cast-iron BBC heads weigh a metric ton, and the choice is a no brainer. Euclidians be damned—round is the new square!

Not one to pass up an opportunity on the dyno, we couldn’t help but cater one test toward the street end of the spectrum. Man does not live by the dragstrip alone, and since most driving is done in the lower third of the rpm range, we wanted to see how much power would be sacrificed with the installation of a dual-plane intake. Off came the Victor Jr. and on went the Performer RPM Air Gap. As expected, the peak power dropped to 602 hp, but we expected the torque to increase. In point of fact, the peak torque was down slightly with the dual-plane to 564 lb-ft, but the Air gap did outperform the single plane up to 4,700 rpm. The choice comes down to where you put the emphasis on power production. For daily street use, the extra torque offered by the dual-plane (as much as 42 lb-ft over the single plane) would offer some serious grins, but those who live by e.t.’s and trap speed will be better served by the single-plane. End

Airflow Numbers

Flow Data: CFM at 28 inches

088 Rec Ports

AFR 265

Lift

Intake

Exhaust

E/I

Intake

Exhaust

E/I

0.050

32

27

84%

37

30

81%

0.100

77

56

73%

73

63

86%

0.200

144

112

78%

164

141

86%

0.300

206

142

69%

241

196

81%

0.400

245

166

68%

298

239

80%

0.500

289

188

65%

332

260

78%

0.600

320

192

60%

331

273

82%

0.700

334

197

59%

320

280

87%

Note how much better the exhaust-to-intake (E/I) improves with the AFR heads. This is where much of the added power originates, along with the AFR’s better mid-lift intake flow.